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2.2. Analogue electronics. Diodes

Semiconductors are materials that conduct electricity under certain conditions. Silicon is the most used to make electronic components. A diode is a semiconductor device that allows current to flow in one direction. It can be used for protection, to block signals, to change AC to DC, etc.
The two leads are called anode (a or +) and cathode (k or -).

If the anode is connected to a higher voltage than the cathode, current will flow from anode to cathode. This is called forward bias.

If the diode is put in the circuit back to front, so that the voltage at the cathode is higher than the voltage at the anode, the diode will not conduct electricity. This is called reverse bias.

 Exercise 29. Look at the diagrams above and fill in the blanks.
The current can only flow from ______ to _______. This direction is called ______ bias.
The current cannot flow from _______ to ______. This direction is called _______ bias.

 Exercise 30. Copy the picture in your workbook and mark the anode and the cathode 

 Exercise 31. Build a circuit with a battery, wires, a bulband a diode. Check how it works. Explain to your partner how you have connected the wires:

The first wire goes from positive lead of the battery to ....


Using diodes to prevent damage to circuits

Diodes are normally used to prevent damage to other polarised components in circuits: eg a diode can protect against current flowing the wrong way if the battery is put in back to front.

An important use of diodes is to prevent circuit damage due to back electromotive force (known as EMF). This is a momentary change in the direction of the flow of electricity when components such as motors, solenoids or relays are switched off. The diode is connected in parallel to the component, which generates the back EMF in reverse bias to the 'normal' direction of flow of electricity in the circuit. A diode used in this way is called a clamping diode.

Reverse biased diode protecting transistor


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